At present approximately 70 interstellar molecules are known. We discuss methods for determining chemical abundances in interstellar clouds and present results for the best studied regions, which include the “spiral arm” clouds seen towards distant continuum sources, quiescent dark and giant clouds, and the gas in regions of active star formation. For many simple molecules abundances are rather uniform over a range of densities and temperatures in quiescent clouds, in accord with gas phase, ion-molecule chemical models. Some striking chemical differences do exist both within and among clouds, however, particularly in star-forming regions. This chapter is organized as follows: 1. Introduction 2. Measurement of the Chemical Composition of Interstellar Clouds 2.1. Introduction 2.2. Optically Thin Emission 2.3. Determination of Total Molecular Column Density 2.4. Multi-Transition Studies 2.5. Optically Thick Emission 2.6. Optical Depth Determination 2.7. Cloud Structure and Molecular Abundances 3. Results 3.1. Identified Interstellar Molecules 3.2. Individual Molecular Clouds 3.2.1. Orion KL 3.2.2. Sgr B2 3.2.3. TMC-1/L134N 3.2.4. Spiral Arm Clouds 3.3. Chemical Abundances 4. Interpretation 4.1. General Uniformities in Abundance 4.2. Chemical Differences Among Clouds 4.2.1. Dark Clouds 4.2.2. Giant Molecular Clouds 4.2.3. Influences of Cloud Temperature 4.3. Chemical Differences Within Clouds 4.3.1. L134N(L183) 4.3.2. Sgr B2 4.3.3. Orion KL 4.3.4. TMC-1 5. Conclusions